1. Field of the Invention
This invention relates to a method for adjusting a focus bias in an optical disc drive, and in particular relates to a method for adjusting a focus bias in an optical disc drive for setting an optimum focusing point.
2. Description of the Prior Art
In optical disc drives which reproduce data from or record and reproduce data to and from optical discs such as CD (compact disc), CD-ROM, CD-R, and CD-RW and the like, an optical beam projected from an optical pick-up is adapted to follow spiral tracks formed in an optical disc by tracking servo, and a distance between a recording surface of the optical disc and an objective lens is kept constant by focus servo, whereby data can be accurately reproduced from or recorded to the optical disc. These servo operations are carried out based on error signals such as a tracking error signal or a focus error signal. These error signals are produced based on distributions of a reflected beam of a laser beam which has been projected to the optical disc. Specifically, the tracking servo signal and the focus servo signal are produced by adding appropriate gain to the error signals and then performing phase compensation thereto. By using the thus produced servo signals, a tracking actuator (that is, a tracking coil) and a focusing actuator (that is, a focusing coil) provided in the optical pick-up are driven through an actuator driver.
In these optical disc drives, it is necessary to make focusing of a laser beam accurately so that a laser beam projected from the optical pick-up can obtain an optimal focusing point at the recording surface of the optical disc. For this purpose, a focusing point adjustment in a focusing direction is carried out, when an optical disc drive is manufactured or shipped (hereinafter, this adjustment is referred to as “focus bias adjustment”).
Conventionally, as shown in publication of Japanese translated version of International Application No. 2002-502534 (pages 5 to 7), such a focus bias adjustment is carried out by reproducing an optical disc under the condition that tracking servo and focus servo are engaged. Then, by using amplitudes of an HF signal obtained under that condition, a focus bias for the focus servo signal is determined so that an amplitude of the HF signal becomes largest, and then such a focus bias is set as the optimum focus bias value, whereby the focus bias adjustment is carried out.
However, in the focus bias adjustment method as described above, there is a case that tracking servo is dislocated during the focus bias adjustment, so that a proper adjustment can not be carried out. Namely, in the method described above, an optimum focusing point is determined by varying the bias for the focus serve within a predetermined adjustment range under the condition that tracking servo is being engaged. However, there is a case that a trouble in which tracking servo is dislocated occurs due to enlargement of a spot diameter of a laser beam during the process of varying the bias for the focus servo. Such a trouble is likely to occur when a focus bias value at the time when the adjustment is carried out is far away from a focus bias value for an optimum focusing point. Further, such a trouble is also likely to occur when an offset component in an error signal extracting circuit is large.